Accidental tourists. That’s what people used to think leatherback sea turtles were. But they were wrong.

“They wanted to be here,” declared Kara Dodge, of the Large Pelagics Research Center of the University of New Hampshire.

What exactly they’re doing here and how they spend their time is the subject of Dodge’s Ph.D. research and she discussed it last week as part of the Wednesday lecture series at Wellfleet Bay Wildlife Sanctuary.

Leatherbacks are not only the largest turtle in the world; they are warm-blooded, at least in part, due to a unique heat exchange system. The warmer arterial blood is blended together with colder blood returning from the animal’s surface before it reaches the heart, so the working core of the animal stays warmer.

“It can keep the body temperature up to 30 degrees higher than the surrounding environment,” Dodge noted. “Leatherbacks can grow to over 6 feet long and weigh 2,000 pounds. They have a high volume to surface ratio and lose heat to the environment slowly.”

They’re fat, which is a good thing.

“They have a blubber layer which is the reason they don’t get cold stunned. That is a heat-conserving adaptation. It is several inches thick, similar to a whale or a dolphin and helps the leatherback insulate itself,” Dodge said.

They don’t mind cold water nearly as much as other turtles and will swim far north of Nova Scotia and dive to frigid depths.

“They can dive to a depth of 4,200 feet, and it’s pretty cold at 4,200 feet,” Dodge noted. “Those are depths that are reached only by sperm whales and elephant seals.”

The other problem with deep dives is the water pressure, but leatherbacks don’t have a hard shell that can be crushed.

“Leatherbacks have a vascularized boney lipid matrix; rows of interlocking bone embedded in a lipid matrix covered with a black skin that gives leatherbacks their name,” Dodge said.

They also have a series of “soft spots” around the body that allow for further flexibility.

“Physiologically they are equipped for depth as well,” explained Dodge. “They are air breathers but are able to store oxygen in the blood and muscle to get around collapsing lungs at great depths.”

Most of their dives are within 650 feet of the sea surface, which is still pretty deep.

What they are doing on these dives is completely known but much of the time they are probably eating. And what does the largest turtle like to eat? Jellyfish.

“Giant leatherbacks subsist entirely on jellyfish. Two of their favorites are moon jellies and lion’s mane jellyfish, which are seasonally abundant here,” Dodge said. “Jellyfish are over 95 percent water and relatively nutrient poor so leatherbacks have to consume large quantities. They have to eat their own body weight each day.”

Page 2 of 3 - That’s a lot of jellyfish but then there are often a lot around, Especially in Nantucket Sound. The turtles have a beak with scissor-like fangs for gripping jellyfish and keratinized spines in their mouth and throat pointing downward to doom for jellyfish.

Dodge is tracking the turtles via battery-operated satellite-linked transmitters. She is also collecting oceanographic data (water temperatures, salinity and such) along with jellyfish densities to determine what the turtle is doing and why. Her three initially tagged turtles, all from Cape Cod Bay last summer, were picked up from entanglements. The turtles get wound up in fishing and lobster gear. She also caught two turtles in Nantucket Sound.

Dodge uses a spotter plane and a specially equipped boat, with a pulpit built by her husband and a giant hoop that’s dropped just in front of the turtle after the boat sneaks up from behind.

Bob, the first turtle, was tracked from Aug. 29 to Sept. 15. It was found dead Sept. 17, just off Dennis entangled in two sets of lobster gear, one trap and a 10-pot trawl. It never left the bay.

Scusset, the second turtle, was tracked for 184 days. It remained in the bay for a month until Oct. 19 when it rounded Race Point, went west to the Mid-Atlantic, swam off the continental shelf, swung by Bermuda following an underwater mountain chain and headed for the Bahamas and U.S. coast when the battery burned out.

Unfortunately, the third turtle was also ill-fated. It had been entangled twice and after a month stopped moving. Dodge tracked it down by kayak in the Chase Garden Creek marsh system on the Dennis/Yarmouth town line. She hauled it in and dissected it.

“We found a 3-foot by 1 1/2-foot piece of plastic sheeting in its stomach,” she said. “It was a very unlucky turtle all around.”

Leatherbacks can mistake plastic bags for jellyfish and once they ingest it it’s stuck in their system.

One of the transmitters, which are fist sized and cost $5,000, quickly stopped working on the first of the Sound-caught leatherbacks. The second turtle, “Holly,” made a beeline for the continental shelf and the transmission stopped after 16 days.

Dodge is looking forward to this summer’s tagging season.

“There are over 40 sightings now of leatherbacks in Rhode Island and Massachusetts waters,” she said. “They’re mostly in Buzzards Bay, Nantucket Sound and some around Martha’s Vineyard. There is a high density of jellyfish off the Vineyard.”

Two weeks ago, on July 17, she captured her first free-swimming non-entangled leatherback, “Henry.”

“He’s an awesome looking turtle. You can see how fat he is,” she said showing off a slide. “When they’re gorging on jellyfish they can blow up. We estimate he is 700 to 800 pounds. In fact the blood work was showing high cholesterol. He has already gone off the continental shelf. In six days since he was tagged he has gone over 450 kilometers. That’s twice as fast as any other tagging data.”

Page 3 of 3 - Henry is a mover but is expected back since this is where the jellyfish are.

Worldwide in the 1980s there were an estimated 115,000 nesting females but that number fell to 20,000 to 30,000 by 1996. Turtles in the Pacific were especially hard hit due to the harvesting of eggs from the nests.

Cape Cod is an important part of their itinerary; 128 leatherbacks stranded here between 1996 and 2006. Dodge’s work will be especially useful in learning about their open water habits and needs.

“This migratory, warm-blooded, largest living reptile is simply too wonderful to abandon to extinction,” herpetologist Skip Lazell wrote in “This Broken Archipelago.”